Marked sexual dimorphism has been discovered in the brains of some birds. The neural system subserving song in the male brain includes at least 3 large nuclei in the telencephalon, 1 nucleus in the medulla and fiber tracts connecting them. In the female zebra and Bengalese finches the three forebrain nuclei are either missing or rudimentary. It is proposed here to explore the possibility of using this unique material as a model for studying the sexual differentiation of brain and behavior. The main aim of this project is to investigate whether the sex differences in the vocal control system of the zebra finch develop under the influence of gonadal hormones. If these sex differences develop according to the general avian pattern of sexual differentiation, early castration of the genetic female should induce the development of a vocal control system, and early injection of estrogen in the genetic male should prevent its development.